Problem with object made from joining objects that both had mirror modifiers
You merged objects mirrored but not moddeled on the same side. You can see that by deactivating the visibility of the modified topo.



to fix that, apply the mirror (in object mode, not edit)



In front ortho view (NumPad1 front view, NumPad5 to switch perspective/ortho view) Box select B one side.



Delete selected vertices with X



add the mirror modifier that should work correctly